@philipncohen That’s probably not a big deal, I admit! I would still be worried about creating a scenario where your silence/refusal to answer such a question for someone else could be interpreted as a bad thing.
On reflection it’s more accurate for me to say I wouldn’t share information about the discussion of their application, good or bad, but I’m not concerned with people knowing I’m on the committee reviewing their application (since my institution makes this information public anyway).
@philipncohen I don't know if it's an ethical reason, but what about this:
A positive, thoughtful tenure letter is of so much value to the recipient that it could feel like an unrepayable favour if they knew with certainty who wrote it.
There is something rare and positive about feeling immense but diffuse gratitude. You can't repay it, so all you can do is pay it forward.
There's my case for "don't tell". Strong opinion, weakly held.
What's the case for disclosing it?